This invention is related to the measurement of bottom hole pressures in deep bore holes in the earth. More particularly, this invention relates to the apparatus and method for filling a conventional type of Bourdon tube pressure measuring instrument with a fluid for communicating well fluid pressure to the Bourdon tube to enable the Bourdon tube to measure well fluid pressures at high temperatures.
It is well known in the art to use a Bourdon tube enclosed in a tubular housing for measuring bottom hole pressures in deep bore holes in the earth. A typical pressure measuring instrument comprises three separate sections. One section includes a recording device in which a sharp jeweled stylus scratches a line trace on the surface of a cylindrical sheet to make a record of the sensed pressure as a function of time. The second section is a pressure sensing section which generally includes a long helically wound Bourdon tube of many terms anchored at its bottom end and fastened to the recording stylus at the upper end. Pressure changes in the hydraulic fluid inside the Bourdon tube cause the upper end to rotate with respect to the fixed bottom end and, therefore, to rotate the stylus against the recording chart. A clock driven mechanism in the recording section moves the chart longitudinally so that a continuous curve is drawn of the pressure as a function of time. The third section of the instrument is devoted to means for contacting the well fluids and transmitting the pressure of the well fluids to the hydraulic liquid in the Bourdon tube. U.S. Pat. No. 3,744,307 to Harper, et al. describes a typical prior art Bourdon tube sensors for use in deep bore holes.
One method of transmitting well fluid pressure to the hydraulic liquid in the Bourdon tube is to contact the well fluids with an extensible bellows, the interior of which is filled with a clean hydraulic liquid and placed in fluid communication with the Bourdon tube. The outside of the bellows contacts the well fluids so that pressure of the well fluid is communicated to the Bourdon tube through the bellows and enclosed hydraulic liquid.
Standard Bourdon tube assembly ordinarily are filled with a liquid such as triethylene glycol, which outgasses at temperatures above 350 degrees Fahrenheit. Outgassing of the liquid in the Bourdon tube seriously degrades instrument accuracy so that it is difficult to obtain meaningful pressure measurements in well bores having ambient temperatures above 350 degrees Fahrenheit. If a bellows is used to transmit well fluid pressures to the Bourdon tube, outgassing of the fluid causes the bellows to expand; and if the temperature is sufficiently high, outgassing of the fluid will rupture the bellows.